Deathscythe will only show up for enemies that are weak. Therefore, this mark has to be conned into revealing itself. First, turn off your healing gambits. Then run around the nearby area (don't go too far!) until one or two of your part members are in critical HP state. Then go to the location of the mark and Deathscythe will appear to take your bait. As soon as it manifests, heal your HP critical teammates quickly. Turn your healing gambits back on and notice that Deathscythe has likely made itself immune to physical attacks. You'll have to rely on magick spells, but that shouldn't be a problem. Exploit its weakness to Holy magick and take this mark down.